# Satellites

A is any object that is in orbit around a planet. There are two types of satellite: natural and artificial. For example, our Moon is a natural satellite, but a communication satellite for relaying signals around the world or a weather satellite are examples of artificial satellites.

The a satellite takes to orbit a planet is called its period. If the satellite is at a high altitude above a planet's surface, it will take a long time to go around – it will have a long period. However if the satellite is closer to the planet's surface, ie it moves to a lower orbit, then it will take less time to go around – it will have a short period.

## Satellite communication

Satellite communication uses ground stations to send and receive signals between artificial satellites which are in orbit around the Earth.

## Geostationary satellites

satellites take 24 hours to orbit the Earth. This is the same time that Earth takes to complete one rotation and so the satellite always remains above the same point on the Earth's surface. To achieve this orbit, the satellite must be at an altitude of $$36,000 km$$ and positioned above the equator of the Earth.

Ground stations send signals to the satellite using a curved dish transmitter to transmit a strong signal. At the satellite the weakened signal is collected by a curved dish receiver. It is then amplified and finally retransmitted, at a different , back to the ground using another curved dish transmitter.

With three geostationary satellites placed in orbit around the equator worldwide communication is permitted. Each satellite communicates with ground stations on different continents.

In satellite television systems the signal from the satellite is broadcast over a wide area and collected by dish aerials on people's homes.